1. Field of the Invention
The present invention relates to a serial printer having a carriage for conveying a print head and, more particularly, to a position detecting apparatus for detecting a current position of the carriage to control the drive timing of the print head.
2. Description of the Related Art
A typical serial printer is arranged as follows. A carriage having a print head thereon is moved parallel to a platen, and the print head is driven according to print data while a paper sheet on the platen is scanned by the print head which moves together with the carriage. Such a printer comprises a position detecting apparatus to control the drive timing of the print head. This position detecting apparatus detects a current position of the carriage while the carriage is moved back and forth according to the rotation of a carriage motor, and generates position data representing the current position of the carriage. The print head is driven upon update of the position data obtained from the position detecting apparatus.
The position detecting apparatus has a linear encoder for generating an output signal which changes in level when a portion of the carriage passes each of coordinate points set in the platen span at a predetermined pitch, a pulse generator for generating a pulse in response to a level change in the output signal from the linear encoder, and a counter for counting the number of pulses generated by the pulse generator. In the initial state, the counter retains, e.g., "1" as position data representing the home position of the carriage. The position data is incremented by "1" in response to a pulse which is generated by the pulse generator when the carriage is moved forward, and is decremented by "1" in response to a pulse which is generated by the pulse generator when the carriage is moved backward.
The position detecting apparatus has the following drawback in a case where the serial printer is of an ink-jet type and performs a bi-directional printing. Generally, a serial printer of this type prints dots by injecting ink from the print head toward a paper sheet. Since the print head cannot immediately inject ink after it is driven, the printing position is deviated due to the movement of the carriage during the time lag.
FIG. 1(a) shows the position of the carriage at which the output signal of the linear encoder changes in level, FIG. 1(b) shows position data output from the position detecting apparatus when the carriage is moved forward, and FIG. 1(c) shows position data output from the position detecting apparatus when the carriage is moved backward. The position detecting apparatus outputs common position data n when the carriage passes a position P1 in its forward movement and when the carriage passes a position P2 in its backward movement. When the printer drives the print head at the timing the position data n is obtained, the print head prints dots at a position R1 or R2. (R1 denotes the position of dots printed in the forward movement of the carriage, and R2 denotes the position of dots printed in the backward movement of the carriage.) Since the printing position of dots changes depending on the moving direction of the carriage, it is difficult to align characters or symbols formed of the printed dots in the paper feed direction.